In internal combustion engines with supercharging, care must be taken than a certain maximum charge pressure not be exceeded; otherwise the engine is endangered both thermally and mechanically. For regulating turbochargers and the engines associated with them, a method and an apparatus are known which make use of a bypass valve often in the form of a charge-pressure regulator for bypassing the exhaust gas turbine; the bypass valve is controlled in accordance with the air pressure in the charge-pressure tube. In this case, a pressure meter on the charge-air side of the compressor acts as a control variable for the charge-pressure regulator. If there is a failure of this charge-pressure regulator, which may be caused by a mechanical defect such as seizing of the bypass valve, then the output limitation for the turbocharger is also absent; the result is that the charge pressure in the intake conduit increases, which accordingly increases both the quantity of the air throughput and the volumetric efficiency. This term is defined as follows from Bosch Dictionary, Vol 2, page 360, "volumetric efficiency is equal to the actual volume of fresh air-fuel charge ratio (or in case of fuel-injection engines, fresh air charge ratio) drawn in at normal atmospheric pressure during each working cycle of the engine, divided by the theoretically possible volume of charge. This ratio is always less than 1." As long as the air flow rate meter is still operating below its maximum signal, then a fuel injection system, for instance, will supply the corresponding supplementary fuel quantity, causing yet a further increase in the volumetric efficiency.
This excessive charge can cause the destruction of an engine within only a few seconds, especially since the malfunctioning status cannot necessarily be recognized by the driver and prevented by closing the throttle valve.
Since present-day turbochargers furnish the maximum permissible charge pressure at approximately half the maximum rpm of the engine, the engine rpm is conventional systems is limited in the case of malfunction to the charge-onset rpm with the aid of an excess-charge-pressure switch. However, as an extra component, the required pressure switch is also a further possible source of malfunctions. This is especially true since a pressure which would cause this switch to respond does not arise during normal operation, and so the switch has to be a product having an extremely long life and extremely good reliability.